1. Field of the Invention
The present invention relates to a method of repairing a light-emitting device, and to a method of manufacturing a light-emitting device using the repairing method in an intermediate process step. More specifically, the present invention relates to a method of repairing a light-emitting device by applying a reverse bias to organic light emitting elements, and to a method of manufacturing a light-emitting device using the repairing method.
The term xe2x80x9clight-emitting devicexe2x80x9d is a generic term referring to organic light emitting displays in which organic light emitting elements formed on a substrate are enclosed between the substrate and a covering material, and to modules in which ICs are mounted in the organic light emitting displays.
2. Description of the Related Art
Organic light emitting elements are self light emitting, and therefore have high visibility. In addition to organic light emitting elements being optimal for making devices thinner because a backlight like that used in liquid crystal display devices (LCDs) is not needed, there is no limit on the angle of view. Light-emitting devices using organic light emitting elements have therefore been in the spotlight in recent years as electro-optical devices that can replace CRTs and LCDs.
Organic light emitting elements have a layer containing an organic compound (hereafter referred to as an organic compound layer) in which luminescence (electroluminescence) is generated by the addition of an electric field, an anode layer, and a cathode layer. Two types of luminescence exist, light emission when returning to a base state from a singlet excited state (fluorescence), and light emission when returning to a base state from a triplet excited state (phosphorescence), and it is possible to apply the repairing method of the present invention to light-emitting devices using either type of light emission.
Note that, in this specification, all layers formed between an anode and a cathode are defined as the organic compound layer. Specifically, light emitting layers, hole injecting layers, electron injecting layers, hole transporting layers, electron transporting layers, and the like are included in the term organic compound layer. Organic light emitting elements basically have a structure in which an anode, a light emitting layer, and a cathode are laminated in the order. Other structures also exist in addition to this structure, such as one in which an anode, a hole injecting layer, a light emitting layer, and a cathode are laminated in the order, and one in which an anode, a hole injecting layer, a light emitting layer, an electron transporting layer, and a cathode are laminated in the order.
Further, making a light emitting element emit light is referred to as driving the light emitting element in this specification. Light emitting elements formed by an anode, an organic compound layer, and a cathode are referred to as organic light emitting elements in this specification.
Organic light emitting elements have high rectification characteristics, and electric current flows in the organic compound if the electric potential of the anode is higher than the electric potential of the cathode. Light is emitted due to recombination of the carrier. Conversely, if the anode is at a lower electric potential than the cathode, almost no electric current will flow in the organic compound layer. From this diode structure, organic light emitting elements are also referred to as organic light emitting diodes (OLEDs).
In making organic light emitting elements, generally one electrode, either the anode of the cathode, is formed, after which the organic compound layer is formed so as to contact the electrode. The remaining electrode is then formed so as to contact the organic compound layer.
The main methods of forming the organic compound layer are a film formation method by evaporation, and a film formation method by spin coating. Whichever method is used, care is taken when performing film formation. Substrates are washed before film formation so that refuse and the like does not adhere to the substrate during film formation of the electrodes and the organic compound layer, and management of the cleanliness of a clean room within which film formation takes place is performed thoroughly.
However, even if every effort is made for cleanliness, there are cases in which refuse and the like adhere to the electrodes or other films, forming holes (pinholes) in the organic compound layer. FIG. 12A shows a simple cross sectional diagram of an organic light emitting element 200 for a case in which there is a short circuit between two electrodes 201 and 202. If a pinhole is opened in an organic compound layer 203, then the two electrodes 201 and 202 are connected in the pinhole when forming the electrode 202 on the organic compound layer 203, thus forming a short circuit in some cases. Note that portions in which two layers formed sandwiching a light emitting layer therebetween are connected through pinholes formed in the light emitting layer are hereafter referred to as defect portions 204.
The voltage-current characteristics of an organic light emitting element that does not have a defect portion are shown in FIG. 13A, and the voltage-current characteristics of an organic light emitting element that has a short circuit in a defect portion are shown in FIG. 13B.
Comparing FIGS. 13A and 13B, it can be seen that the electric current flowing in the organic light emitting element 200 when a reverse bias voltage is applied to the organic light emitting element 200 is larger for the case of FIG. 13B.
This is because the two electrodes are shorted in the defect portion 204 for the case of FIG. 13B, and current therefore flows in the defect portion 204, differing from FIG. 13A.
If the two electrodes 201 and 202 are shorted in the defect portion 204, then the brightness of light emitted from the organic compound layer drops. The electric current flowing in an organic light emitting element having a defect portion when a forward bias voltage is applied is shown schematically in FIG. 12B.
It can be considered that the defect portion 204 has a resistance RSC therein, and thereby the two electrodes of the organic light emitting element 200 are connected for cases in which the two electrodes 201 and 202 are shorted in the defect portion 204. Consequently, if the electric current flowing in the defect portion 204 is taken as ISC, and the electric current flowing in the organic compound layer 203 is taken as Idio when an electric current Iori flows from one electrode of the organic light emitting element, then the electric current Iori satisfies the equation Iori=ISC+Idio.
If Iori is taken as fixed in the aforementioned equation Iori=ISC+Idio, then the current Idio actually flowing in the organic compound layer 203 becomes smaller in organic light emitting elements having defect portions. If the resistance RSC in the defect portion 204 becomes smaller, then ISC increases, and therefore this tendency becomes more conspicuous and the rectification properties of the organic light emitting element 200 collapse.
If the current Idio flowing in the organic compound layer 203 becomes smaller, then the brightness of light emitted from the organic light emitting element 200 drops. That is, if there is a short circuit in the defect portion, then the brightness of light emitted from the organic light emitting element when a forward bias voltage is applied becomes lower compared to a case in which there is no short circuit.
Further, even if the organic compound layer is formed by laminating together a plurality of layers, if a pinhole is formed in the light emitting layer, then the hole injecting layer or the hole transporting layer will be connected to the electron injecting layer or the electron transporting layer through the pinhole. Portions in which the hole injecting layer or the hole transporting layer is connected to the electron injecting layer or the electron transporting layer also become the source of lowered brightness in light emitted from the organic light emitting element because a reverse bias electric current flows, similarly to that in the defect portion in which the electrodes are shorted. Note that, all portions in which two layers, formed sandwiching a light emitting layer therebetween, are connected through a pinhole formed in the light emitting layer are hereafter referred to generically as defect portions. The defect portions are portions in which an anode and a cathode are electrically short-circuited.
In addition, if there is a short circuit in the defect portion, then not only will the brightness of light emitted from the organic light emitting element drop, but the degradation of the organic compound layer in the vicinity of the defect portion will be promoted because electric current always flows in the defect portion.
In view of the above problems, an object of the present invention is to propose a method of repairing a defect portion.
The applicants of the present invention consider that even if a defect portion is formed in an organic light emitting element, a drop in the electric current flowing in an organic compound layer when a forward bias voltage is applied can be prevented, provided that the resistance in the defect portion is increased.
A method of increasing the resistance RSC in the defect portion by applying a reverse bias voltage to the organic light emitting element, causing a reverse bias electric current Irev to flow, is proposed.
If the reverse bias electric current Irev flows in the organic light emitting element, then almost all of this current flows in the defect portion that has a short circuit, and almost none flows in the organic compound layer. If an electric current ISC flowing in the defect portion becomes larger, then the temperature of the defect portion increases, and therefore the defect portion burns out, is gasified and vaporizes, or is oxidized or carbonized and becomes an insulator. A certain change occurs in the defect portion, and as a result, the resistance RSC therefore becomes larger. Note that, in this specification, the defect portion, after the resistance RSC has been increased by the flow of a reverse bias electric current, is referred to as a denatured layer.
If the resistance RSC becomes larger, then the electric current flowing in the denatured layer becomes smaller when a forward bias voltage is applied to the organic light emitting element. Instead, the electric current flowing in the organic compound layer becomes larger, and the brightness of emitted light increases.
Further, degradation of the organic compound layer existing in the vicinity of the defect portion is easily promoted because current always flows in the defect portion. However, the denatured layer has a high resistance RSC, and therefore current does not easily flow. The promotion of degradation of the organic compound layer existing in the vicinity of the denatured layer can thus be prevented.
Materials for cathodes of the organic light emitting elements used in the present invention are considered next. It is preferable that a material having a low work function be used for the cathode because electron injection is performed there. Li and Mg are included in such materials, and their influence on the light emitting element was investigated.
The C-V (capacitance-voltage) characteristics were investigated using a test piece having a MOS (metal oxide silicon) structure. The ratio between a MOS capacitance C and an insulating film capacitance Cox depends upon voltage. When a clean MOS transistor having no impurities was used, C/Cox showed an ideal C-V characteristic unambiguously determined in response to voltage. Cases in which there is a shift from this ideal C-V characteristic show that the MOS transistor is contaminated by ionic impurities.
The initial characteristics, and the characteristics after applying a 1.7 MV/cm bias to the MOS transistor at a temperature of 150xc2x0 C. for one hour were measured. A process in which a bias is applied while adding thermal shock is referred to as BT (bias temperature) processing. Taking Si as a base level, positive bias application processing (+BT processing) and negative bias application processing (xe2x88x92BT processing) were performed.
Measurement of the C-V characteristics was performed while increasing the electrode electric potential from xe2x88x9210V to +10V, and then decreasing the electrode electric potential from +10V to xe2x88x9210V, taking Si as the base level.
For the MOS structure, a silicon oxide film having a 50 nm film thickness was formed on a silicon substrate, and in addition, AlMg, MgAg, or AlLi was formed as an electrode on the silicon oxide film. Note that, for the measurements, the weight ratios were as follows: AlMg, Al:Mg=95:5; MgAg, Mg:Ag=90:10; and AlLi, Al:Li=90:110.
A case of using AlMg as the electrode (FIG. 17) and a case of using MgAg as the electrode (FIG. 18) show ideal characteristics in which C/Cox is unambiguously determined by voltage for both the initial characteristics, and the characteristics after performing BT processing. This shows that the diffusion of Mg is at a level that can be ignored even after imposing the thermal shock.
However, for a case in which AlLi was used as the electrode (FIG. 19), the resulting C-V characteristics show a large shift compared to the ideal values. In particular, a characteristic 600 showed a large variation with respect to the ideal value after performing positive bias application BT processing (+BT processing). It is thought that this is because Li+ elutes from the electrode and diffuses due to electrical repulsion when the AlLi electrode has a positive electric potential. A hysteresis can also be seen in the C-V characteristics for the initial characteristics, and for the characteristics after performing negative bias application processing (xe2x88x92BT processing). This may be due to a very small amount of Li+ that elutes by the application of a positive voltage during measurement.
The results thus show that electrodes to which high diffusion Li is added are not preferable for the structure of the present invention in which the cathode is given a positive electric potential when applying a reverse bias to the organic light emitting elements.
If a reverse bias is applied and Li+ contained in the cathode elutes, then the device will become contaminated while the defect portion is being repaired by the reverse bias application. The Li+ that elutes from the cathode has high mobility, and therefore may penetrate an interlayer insulating film, reach a TFT, and contaminate a channel layer. The TFT performance thus degrades.
Of course, structures in which high mobility Na, not only Li, is contained in the cathode are also not preferable for a similar reason. The less the amount of Li and Ni contained in the cathode becomes, the better.
Further, it can be seen that it is preferable to use electrodes that contain low diffusion Mg. For example, the use of AlMg and MgAg is extremely effective.
The organic compound layer in the light emitting element of the present invention can be formed using known organic compound materials. The organic compound layer may also be formed having an inorganic material as a portion of the organic compound layer. For example, by forming the organic compound layer using as a portion a layer that contains a low work function alkaline metal element, or an alkaline earth metal element, an increase in the injection characteristics of electrons from the cathode can be achieved. In addition, light emitting elements having superior characteristics can be formed by including an organic material capable of increasing the transportability of the injected carrier in the organic compound layer. Note that there are no particular limitations placed on the type of inorganic materials capable of being included in the organic compound layer, nor are there limitations placed on the arrangement of layers containing inorganic materials within the organic compound layer. Known inorganic materials can be freely disposed.
Note that it is also possible to use the present invention in passive type light-emitting devices, not only in active matrix type light-emitting devices.
Hereinbelow, a description is given on the structure of the present invention.
According to the present invention, there is provided a method of repairing a light-emitting device containing an organic light emitting element having an anode, a cathode, and an organic compound layer sandwiched between the anode and the cathode, characterized in that:
the concentration of at least one of Li and Na contained in the cathode is equal to or less than 1xc3x971018 atoms/cm3,
the repairing method comprising:
applying a reverse bias voltage between the anode and the cathode.
According to the present invention, there is provided a method of repairing a light-emitting device containing an organic light emitting element having an anode, a cathode, and an organic compound layer sandwiched between the anode and the cathode, characterized in that:
the concentration of at least one of Li and Na contained in the cathode is equal to or less than 1xc3x971018 atoms/cm3,
the repairing method comprising:
applying a reverse bias voltage between the anode and the cathode, thus making an electric current flow in portions at which the anode and the cathode are electrically short-circuited, causing the short-circuited portions to emit heat; and
making the portions that emit heat into higher resistance portions, or insulating portions.
According to the present invention, there is provided a method of repairing a light-emitting device containing an organic light emitting element having an anode, a cathode, and an organic compound layer sandwiched between the anode and the cathode, characterized in that:
the concentration of at least one of Li and Na contained in the cathode is equal to or less than 1xc3x971018 atoms/cm3; and
the organic compound layer has a hole injecting layer, a hole transporting layer, an electron injecting layer or an electron transporting layer, and a light emitting layer,
the repairing method comprising:
applying a reverse bias voltage between the anode and the cathode, thus making an electric current flow in portions at which a layer on the light emitting layer and a layer under the light emitting layer are electrically short-circuited, causing the short-circuited portions to emit heat; and
making the portions that emit heat into higher resistance portions, or insulating portions.
According to the present invention, there is provided a method of repairing a light-emitting device containing an organic light emitting element having an anode, a cathode, and an organic compound layer sandwiched between the anode and the cathode, characterized in that:
the concentration of at least one of Li and Na contained in the cathode is equal to or less than 1xc3x971018 atoms/cm3,
the repairing method comprising:
applying a reverse bias voltage between the anode and the cathode, thus making at least one of the anode and the cathode penetrate into the organic compound layer, causing an electric current to flow in portions at which the anode and the cathode are in electrical contact.
According to the present invention, there is provided a method of repairing a light-emitting device, characterized in that there is the electrical current flowing in portions at which the anode and the cathode are in electrical contact, causing heat to be emitted, and making the portions that emit heat into higher resistance portions, or insulating portions.
According to the present invention, there is provided a method of repairing a light-emitting device containing an organic light emitting element having an anode, a cathode, and an organic compound layer sandwiched between the anode and the cathode, characterized in that:
the concentration of at least one of Li and Na contained in the cathode is equal to or less than 1xc3x971018 atoms/cm3; and
the organic compound layer has a hole injecting layer, a hole transporting layer, an electron injecting layer or an electron transporting layer, and a light emitting layer;
the repairing method comprising:
applying a reverse bias voltage between the anode and the cathode, thus making a layer on the light emitting layer or a layer under the light emitting layer penetrate into the light emitting layer, causing an electric current to flow in portions at which the layer on the light emitting layer and the layer under the light emitting layer are in electrical contact.
According to the present invention, there is provided a method of repairing a light-emitting device, characterized in that there is the electrical current flowing in portions at which the layer on the light emitting layer and the layer under the light emitting layer are in electrical contact, causing heat to be emitted, and making the portions that emit heat into higher resistance portions, or insulating portions.
According to the present invention, there is provided a method of repairing a light-emitting device, characterized in that the cathode is an alloy containing at least one of Be, Mg, Ca, Sr and Ba.
According to the present invention, there is provided a method of repairing a light-emitting device, characterized in that the cathode is an alloy containing magnesium.
According to the present invention, there is provided a method of repairing a light-emitting device, characterized in that the cathode is AlMg, MgAg or MgAgAl.
According to the present invention, there is provided a method of repairing a light-emitting device containing an organic light emitting element having an anode, a cathode, and an organic compound layer sandwiched between the anode and the cathode, characterized in that:
the cathode contains Mg, and at least one of Al and Ag,
the repairing method comprising:
applying a reverse bias voltage between the anode and the cathode.
According to the present invention, there is provided a method of repairing a light-emitting device containing an organic light emitting element having an anode, a cathode, and an organic compound layer sandwiched between the anode and the cathode, characterized in that:
the cathode contains Mg, and at least one of Al and Ag,
the repairing method comprising:
applying a reverse bias voltage between the anode and the cathode, thus causing an electric current to flow in portions at which the anode and the cathode are electrically short-circuited, causing the short-circuited portions to emit heat; and
making the portions that emit heat into higher resistance portions, or insulating portions.
According to the present invention, there is provided a method of repairing a light-emitting device containing an organic light emitting element having an anode, a cathode, and an organic compound layer sandwiched between the anode and the cathode, characterized in that:
the cathode contains Mg, and at least one of Al and Ag; and
the organic compound layer has a hole injecting layer, a hole transporting layer, an electron injecting layer or an electron transporting layer, and a light emitting layer,
the repairing method comprising:
applying a reverse bias voltage between the anode and the cathode, thus making an electric current flow in portions at which a layer on the light emitting layer and a layer under the light emitting layer are electrically short-circuited, causing the short-circuited portions to emit heat; and
making the portions that emit heat into higher resistance portions, or insulating portions.
According to the present invention, there is provided a method of repairing a light-emitting device containing an organic light emitting element having an anode, a cathode, and an organic compound layer sandwiched between the anode and the cathode, characterized in that:
the cathode contains Mg, and at least one of Al and Ag,
the repairing method comprising:
applying a reverse bias voltage between the anode and the cathode, thus making at least one of the anode and the cathode penetrate into the organic compound layer, causing an electric current to flow in portions at which the anode and the cathode are in electrical contact.
According to the present invention, there is provided a method of repairing a light-emitting device, characterized in that there is the electrical current flowing in portions at which the anode and the cathode are in electrical contact, causing heat to be emitted, and making the portions that emit heat into higher resistance portions, or insulating portions.
According to the present invention, there is provided a method of repairing a light-emitting device containing an organic light emitting element having an anode, a cathode, and an organic compound layer sandwiched between the anode and the cathode, characterized in that:
the cathode contains Mg, and at least one of Al and Ag; and
the organic compound layer has a hole injecting layer, a hole transporting layer, an electron injecting layer or an electron transporting layer, and a light emitting layer,
the repairing method comprising:
applying a reverse bias voltage between the anode and the cathode, thus making a layer on the light emitting layer or a layer under the light emitting layer penetrate into the light emitting layer, causing an electric current to flow in portions at which the layer on the light emitting layer and the layer under the light emitting layer are in electrical contact.
According to the present invention, there is provided a method of repairing a light-emitting device, characterized in that there is the electrical current flowing in portions at which the layer on the light emitting layer and the layer under the light emitting layer are in electrical contact, causing heat to be emitted, and making the portions that emit heat into higher resistance portions, or insulating portions.
According to the present invention, there is provided a method of repairing a light-emitting device, characterized in that the concentration of at least one of Li and Na contained in the cathode is equal to or less than 1xc3x971018 atoms/cm3.
According to the present invention, there is provided a method of repairing a light-emitting device, characterized in that the amount of Mg contained in the cathode is equal to or more than 1xc3x971020 atoms/cm3.
According to the present invention, there is provided a method of repairing a light-emitting device, characterized in that the reverse bias voltage is applied during fixed periods of time.
According to the present invention, there is provided a method of repairing a light-emitting device, characterized in that the reverse bias voltage is gradually increased during the application until it settles within xc2x115% of the height at which an avalanche current begins to flow within the organic compound layer.
According to the present invention, there is provided a method of repairing a light-emitting device, characterized in that the organic light emitting elements are arranged in a matrix shape, and a thin film transistor is connected to each of the organic light emitting elements.
According to the present invention, there is provided a method of manufacturing a light-emitting device containing an organic light emitting element having an anode, a cathode, and an organic compound layer sandwiched between the anode and the cathode, characterized by comprising:
forming the cathode such that the concentration of at least one of Li and Na contained in the cathode is equal to or less than 1xc3x971018 atoms/cm3; and then
applying a reverse bias voltage between the anode and the cathode.
According to the present invention, there is provided a method of manufacturing a light-emitting device containing an organic light emitting element having an anode, a cathode, and an organic compound layer sandwiched between the anode and the cathode, characterized by comprising:
forming the cathode such that the concentration of at least one of Li and Na contained in the cathode is equal to or less than 1xc3x971018 atoms/cm3; and then
applying a reverse bias voltage between the anode and the cathode, thus making an electric current flow in portions at which the anode and the cathode are electrically short-circuited, causing the short-circuited portions to emit heat, and making the portions that emit heat into higher resistance portions, or insulating portions.
According to the present invention, there is provided a method of manufacturing a light-emitting device containing an organic light emitting element having an anode, a cathode, and an organic compound layer sandwiched between the anode and the cathode, characterizing by comprising:
forming the cathode from an alloy containing Mg and at least one of Al and Ag; and then
applying a reverse bias voltage between the anode and the cathode.
According to the present invention, there is provided a method of manufacturing a light-emitting device containing an organic light emitting element having an anode, a cathode, and an organic compound layer sandwiched between the anode and the cathode, characterizing by comprising:
forming the cathode from an alloy containing Mg and at least one of Al and Ag; and then
applying a reverse bias voltage between the anode and the cathode, thus making an electric current flow in portions at which the anode and the cathode are electrically short-circuited, causing the short-circuited portions to emit heat, and making the portions that emit heat into higher resistance portions, or insulating portions.